The invention is primarily concerned with the production of hollow bodies which are gas tight and which can be formed into intricate shapes such as is required for the manufacture of high temperature heat exchangers. One of the requirements for such heat exchangers (particularly where one side of the heat exchanger is exposed to highly corrosive gases such as coal-fired combustion gases and the other side of the heating exchanger contains an inert gas used for a gas turbine in a power plant) is that the heat exchanger surfaces be impermeable to penetration by either of the gases which it serves to isolate. While dense silicon carbide pieces have been provided for diffusion furnaces as described in the co-pending application of Alliegro and Coes, Ser. No. 530,083 filed Dec. 6, 1974, U.S. Pat. No. 3,951,587, so far as is known, no composite structures have been created which provide complex hollow shapes where separate cast pieces of silicon carbide are joined to form the unitary gas tight structure which is the principal objective of the present invention.